Generally, with a radio device, low power consumption and an accurate transmission power control are required. By reducing power consumption, a radio device that mounts a battery, for example, a mobile telephone is able to communicate longer. Also, by carrying out an accurate transmission power control, it is possible to increase communication quality. Absence of accurate transmission power control becomes a cause of increased noise leakage against other radio devices, so that in many cases an accurate transmission power control is strictly required by, for example, a radio communication standard.
As a preferable transmitting technology to achieve low power consumption, conventionally, a polar modulation scheme has been proposed (for example, see patent literature 1).
FIG. 1 shows an example of a typical transmission apparatus using a polar modulation scheme. The transmission apparatus has polar signal generation circuit 1, amplitude control circuit 2, phase-modulated signal generation circuit 3, and power amplifier (hereinafter “PA”) 4. In this transmission apparatus, polar signal generation circuit 1 generates an amplitude component signal and a phase component signal of a modulated transmission signal from an input signal (that is, the modulated transmission signal). Amplitude control circuit 2 controls a power supply voltage to be supplied to PA 4 based on the amplitude component signal, and phase-modulated signal generation circuit 3 generates a phase-modulated signal to be input to PA 4 based on the phase component signal.
This transmission apparatus secures the dynamic range of transmission power by switching PA 4 between compressed mode and uncompressed mode. Further, compressed mode may be rephrased as saturation operation mode and uncompressed mode as non-saturation operation mode.
This transmission apparatus operates PA 4 in compressed mode when high transmission power is required. On the other hand, when low transmission power is required, the transmission apparatus operates PA 4 in uncompressed mode. Specifically, in compressed mode, the transmission apparatus controls the output power by changing the level of the power supply voltage to PA 4 according to desired output power. Given that the PA operates in the saturation region, this compressed mode is inherently very accurate with respect to output power.
On the other hand, in uncompressed mode, the transmission apparatus controls the output power by changing the level of the phase-modulated signal to be input to PA 4 according to desired output power. In this uncompressed mode, given that the PA operates in the linear region, the output power can not be controlled as accurately as in compressed mode.